A technique recently published for enhancing the gain of MOS common-source amplifier stages is the "regulated cascode". To see the development of this technique, first look at a simple common-source stage as shown in FIG. 1. As shown in this drawing, r.sub.ds1 is the drain-source conductance inherant to device m.sub.1, and R.sub.L is the external load resistance. If r.sub.ds1 were not present, the gain of this amplifier could be made arbitrarily high by selecting arbitrarily large values of R.sub.L, since in this case EQU V.sub.out /V.sub.in =-g.sub.m1 .times.R.sub.L
But, with r.sub.ds1 in place, EQU V.sub.out /V.sub.in =-g.sub.m1 .times.(r.sub.ds1 .parallel.R.sub.L)
and the gain would be limited to at most -g.sub.m1 r.sub.ds1.
Physically, this is because the small signal voltage across r.sub.ds1 is the same as the output voltage, and this causes a small signal current to flow through r.sub.ds1. This diverts some of the current g.sub.m1 V.sub.in from the device transconductance away from R.sub.L, reducing the small signal output voltage developed.
A widespread circuit technique for gain enhancement is the cascode configuration shown in FIG. 2. In this circuit, a current i.sub.s2 is flowing into the source of MOS transistor m.sub.2. The impedance to small-signal ground from the source of transistor m.sub.2 is approximately 1/g.sub.m2 across r.sub.ds1, and KCL(Kirchoff's current law) at the source of transistor m.sub.2 gives: EQU g.sub.m1 V.sub.in i.sub.s2 +(i.sub.s2 /g.sub.m2)/r.sub.ds1 =0
but, EQU i.sub.s2 =V.sub.out /R.sub.L
therefore EQU g.sub.m1 V.sub.in +V.sub.out /R.sub.L +V.sub.out /g.sub.m2 R.sub.L r.sub.ds1 =0
or ##EQU1## compared to a simple common-source amplifier, the gain is now only limited by the parallel combination of R.sub.L and (g.sub.m2 R.sub.L)r.sub.ds1 instead of R.sub.L and r.sub.ds1. Therefore, the effective drain-source resistance of transistor m.sub.1 has been multiplied by g.sub.m2 R.sub.L (the "gain" of transistor m.sub.2) and much higher gains may be realized by increasing R.sub.L. Thus, the basic idea of the cascode circuit is to clamp the drain voltage of amplifier device m.sub.1 independent of the output voltage which is the drain of cascode device m.sub.2, but still allow whatever drain current generated by device m.sub.1, or controlled by the input voltage, to flow through to the load resistance to develop an output voltage. Thus, by clamping the voltage at the drain of device m.sub.1, there is substantially zero incremental small signal current flowing through r.sub.ds1.
In FIG. 3, there is shown a schematic diagram of the "regulated cascode". This circuit is an improved version of the cascode, with the addition of a feedback amplifier. The additional feedback amplifier has voltage gain A, and V.sub.D1 is its bias voltage that sets the desired level at the drain of transistor M.sub.1. Now the impedance to ground that current i.sub.s2 sees is approximately 1/g.sub.m2 A instead of 1/g.sub.m2. This "regulates" the voltage across r.sub.ds1 to a smaller small-signal value by a factor of A, resulting in smaller current through r.sub.ds1. For this stage V.sub.out /V.sub.in =-g.sub.m1 (R.sub.L .parallel.(g.sub.m2 R.sub.L A)r.sub.ds1). The effective r.sub.ds of transistor m.sub.1, which is scaled by the effective "gain" of transistor m.sub.2, has been increased by a factor A, and the gain of this stage may be raised over that of the simple cascode by increasing resistor R.sub.L. Thus, the "regulated cascode" regulates the voltage at the drain of device m.sub.1 to a precise figure. It's not a perfect voltage source but it is better by a factor of A over the simple cascode circuit described above.
The current invention is an improved circuit topology for realizing the feedback amplifier in a "regulated cascode". There are two recent publications that describe two different approaches for the "regulated cascode", each with a drawback. A first paper by Sachinger and Guggenbuhl, in IEEE JSSC, Feb. 1990, p. 289, characterizes a regulated cascode where the feedback amplifier 11 as shown in FIG. 4, is a simple MOS common-source gain stage. Transistors m.sub.1 and m.sub.2, as before are the primary common-source amplifier and cascode devices, and transistor m.sub.3 is the feedback amplifier, biased with current source m.sub.4.
The drawback to this approach is that the effective value of V.sub.D1 with this feedback amplifier, or the value of the inverting input for which the amplifier is in its high-gain region, is V.sub.Gs3 (the voltage across the gate and source of transistor m.sub.3). This means that the feedback amplifier will regulate the drain voltage of transistor m.sub.1 to be at a DC bias level of typically 1.5 volts above the negative supply rail. It is important to preserve the gain-enhancement operation over as wide a swing in the output voltage as possible, and with such a high voltage at the drain of transistor m.sub.1, the output voltage need only go below perhaps 2 volts to send transistor m.sub.2 into the ohmic region, degrading the stage gain significantly. High gain in this "regulated cascode" circuit depends on both transistors m.sub.1 and m.sub.2 being in saturation, and transistor m.sub.1 will remain in saturation with only V.sub.Gs1 (the voltage from the gate to source of transistor m.sub.1)-V.sub.T1 (the threshold voltage of transistor m.sub.1), as a bias level at its drain. It need not have as much as V.sub.Gs3. If the effective V.sub.D1 of the feedback amplifier can be set at V.sub.Gs1 -V.sub.T1 or perhaps 0.5 volts, then the output voltage may swing significantly lower than 2 volts, perhaps 1 volt, before transistor m.sub.2 is brought into the ohmic region. This is possible with the feedback amplifier designed by Bult and Geelen, as described in the ISSCC digest 1990, p. 105, and as shown in FIG. 5. In this circuit transistors m.sub.10 and m.sub.11 act as a differential pair and are the differential transconductance in the feedback amplifier. Transistors m.sub.12 and m.sub.13 act as current sources that supply bias current for the differential pair m.sub.10 and m.sub.11 and also supply bias current for folded cascode devices comprising transistors m.sub.14 and m.sub.15, transistors m.sub.16 and m.sub.17, and transistors m.sub.18 and m.sub.19. Transistors m.sub.17 and m.sub.19 form diode connected transistors and they take the drain current of transistor m.sub.15 and mirror it down as a drain current of transistor m.sub.18. So basically, transistors m.sub.16, m.sub.17, m.sub.18, and m.sub.19 are one P-channel current mirror. By mirroring the drain current of transistor m.sub.15 down into the drain of transistor m.sub.14, a differential to single-ended conversion occurs where the differential transconductance of transistor pair m.sub.10 and m.sub.11 make a differential current into the sources of transistors m.sub.14 and m.sub.15. The difference of that current is what appears as the current into the node at the junction of the drains of transistors m.sub.14 and m.sub.16 which acts as a high impedance because of the stacked cascode devices. Thus, the voltage gain at this output terminal that feeds the gate of transistor m.sub.2 will be a gain of 100 for a 1 micron process from the differential voltage input to transistors m.sub.10 and m.sub.11 to the single-ended voltage that you use to feed the gate of transistor m.sub.2. Transistor m.sub.20 acts as a tail current source for the differential pair m.sub.10 and m.sub.11. Transistors m.sub.12 and m.sub.13 are current sources that will both absorb the current coming through transistors m.sub.10 and m.sub.11 and also draw additional current through the cascode devices, i.e. transistors m.sub.14 and m.sub.15. With this much more elaborate feedback amplifier as shown in FIG. 4, V.sub.D1 may be set to any desired level, close to the negative supply rail, and the signal swing at V.sub.out for which gain enhancement is preserved is greatly increased. However, the drawback of this approach is the large chip area and power required by such an elaborate amplifier.
It is therefore a general object of the present invention to provide a feedback amplifier section for a regulated cascode circuit which saves chip area and requires less power to operate.
It is a more specific object of the present invention to provide a feedback amplifier section for a regulated cascode circuit which requires only 5 transistors to implement.
It is a further object of the present invention to provide a feedback amplifier section for a regulated cascode circuit wherein the value of V.sub.D1 may be set to any desired level, close to the negative supply rail, and the signal swing at V.sub.out for which gain enhancement is preserved is greatly increased.
Other objects of the invention will become apparent to those of ordinary skill in the art having reference to the following specification, in conjunction with the drawings.